There is a high co-morbidity between sleep apnea and essential hypertension. This study will test the hypothesis that apneic episodes predispose to hypertension by repeated nighttime arousals that raise blood pressure to daytime levels via activation of the sympathetic nervous system and release of stress hormones. Plasma and urinary catecholamines, renin, neuropeptide Y, atrial natriuretic factor, blood pressure, and cortisol will be examined during sleep, resting wakefulness, and after mild behavioral stressors. Beta adrenergic receptors on lymphocytes and the response to adrenergic agonists will be characterized. This study will determine if apneic hypertensives have an excessive response to the stress of hypoxia or to combined hypoxia plus breathholding. Mild hypoxia will be induced, and the effects of hypoxia will be examined on norepinephrine metabolism, baroreceptor functioning, and sensitivity to infused isoproterenol. Eighty unmedicated patients will be studied: 20 hypertensives with sleep apnea, 20 hypertensives without apnea, 20 normotensives with sleep apnea, and 20 normotensives without apnea. All patients will be admitted for a 2 day stay in the Clinical Research Center.